1. Field of the Invention
The invention relates to a display drive circuit for operating a display device, which uses an organic electroluminescent device (hereinafter called “an organic EL device”) or a light emitting diode (hereinafter called “an LED”), and specifically, relates to a display drive circuit formed in a chip manufactured by a Chip On Glass (hereinafter called “a COG”) implementation technology, which connects lead lines for a display formed on a glass substrate by bonding.
2. Description of the Related Art
FIG. 2A is a diagram showing the entire skeleton framework of a display panel having an organic EL drive circuit formed on a COG, and FIG. 2B is a sectional view taken on line I1-I2 of FIG. 2A.
The display panel includes a nearly square-shaped glass substrate 2, and a display area 10 displaying the images is located in the center of the substrate 2. In the display area 10, a plurality of data lines SEG and a plurality of scanning lines COM, which are perpendicular to the data lines SEG, are formed. An organic EL device 11 is formed at each intersection of the data lines SEG and the scanning lines COM so that the organic EL devices 11 are arranged in a lattice-like manner. Both of the data lines SEG and the scanning lines COM extend from the display area 10 to an edge of the glass substrate 2 as lead lines. Each of the data lines SEG and the scanning lines COM are formed of a transparent conductive layer using ITO (indium Tin Oxide) for instance. The transparent conductive layer using ITO has a large wiring resistance, compared to that in a wiring layer made of cupper.
A display drive circuit according to the related art includes a plurality of data line drive circuits 20-1 . . . 20-n and a plurality of scanning line drive circuit 30-1 . . . 30-n. Each of the data line drive circuits 20-1 . . . 20-n is formed on one of the lead lines of the data lines SEG extending from the display area 10, and is formed in an individual chip, which is implemented by the COG method. Each data line drive circuit 20 includes switching elements such as transistors, which are operated in response to image data for displaying the images, and which have a function to supply a predetermined electric current to each of the data lines SEG. Also provided on each of the lead lines of the scanning lines COM extending from the display area 10 is a respective one of the scanning line drive circuits 30-1 . . . 30-n, each formed in an individual chip, which is implemented by the COG method. Each scanning line drive circuit 30 includes switching elements such as transistors, which are operated in response to image data for displaying the images, and which have a function for supplying ground electric potential (ex. 0 volt) to the scanning lines SEG.
The glass substrate is equipped with unillustrated electrical components, such as a control circuit, in an area around the display area 10.
FIG. 3 is a skeletal circuit diagram in an area X of the FIG. 2A, which is adjacent to one of the organic EL devices 11, and in an area Y of the FIG. 2A, which is a part of one of the scanning line drive circuit 30. FIG. 4A is a diagram showing a skeleton framework of one of the scanning line drive circuit 30 shown in FIG. 2A, and FIG. 4B is a sectional view taken on line I21-I22 of FIG. 4A.
As shown in FIG. 2A, the organic EL device is connected in a forward direction between a data line SEG and a scanning line COM that is perpendicular to the data line in the area X.
As shown in FIGS. 2A and 2B and FIGS. 4A and 4B, each of the scanning line drive circuit 30 includes a rectangularly-shaped substrate 31. An elongated ground line 32 having a width W and length L is formed on the substrate 31 along the longer side of the substrate 31. A plurality of output terminals 33-1 . . . 33-n are disposed on the substrate 31 along one of the longer side of the substrate 31, and a plurality of a switches 34-1 . . . 34-n, each of which includes a transistor, are formed on the substrate 31 wherein each of the switches 34 is disposed between one of the output terminals 33-1 . . . 33-n and the ground line 32. The switches 34 are operated by the control circuit.
At both ends of the ground line 32, two ground terminals 35-1 and 35-2 are formed near the opposite longer side of the substrate 31. Each output terminal 33 is connected to one of the scanning lines through a bump electrode 36 formed thereon, and each of the two ground terminals is grounded through another bump electrode 36 formed thereon.
One of the organic EL devices 11 (for example the organic EL device 11 illustrated in FIG. 2A) emits light in the following way. Initially, the scanning line drive circuit 30-1, which connects the organic EL device 11 to be eliminated, is connected to the ground line 32 through the output terminal 33-1 and the switch 34-1 so that the ground electric potential (0 volt) is supplied to the scanning line COM connected to the organic EL devices 11. Then, the drive current from the data line drive circuit 20-1 is supplied to the data line SEG, which is connected to the organic EL devices 11. Then, the drive current flows in the flowing order; the data line SEG→the organic EL device 11→the scanning line COM→the output terminal 33-1→switch 34-1→the ground line 32→the ground terminals 34-1 and 34-2. As a result of the flow of the drive current as described above, the organic EL device 11 emits light. The light intensity of the organic EL device 11 depends on the value of the drive current.
Some technologies relating to the display panel having a configuration similar to that described above are disclosed in the following publications.
Reference 1: Japanese laid open patent 2002-151276
Reference 2: Japanese laid open patent 2003-131617
Reference 3: Japanese laid open patent 2004-206056
Reference 4: Japanese laid open patent 2005-144685
According to the reference 1, an EL display device with a good balance between colors of EL elements and with a good balance in emission intensity, which is capable of displaying brightly hued images, is disclosed. According to the reference 2, EL drive circuits, which are similar to the drive circuits of FIGS. 2A, 2B and 3, are disclosed. According to the reference 3, EL drive circuits, which suppress luminance unevenness and retain display quality without enlarging a frame part, are disclosed. Further, the reference 4 discloses line heads of the configuration without any difference in the quantity of light emitted from a plurality of light emitting elements, and an image forming apparatus using the same.
The following problems are recognized in the display drive circuit, for example, the scanning line drive circuits 30, in the related art.
In such a scanning line drive circuits 30, as described above, two ground terminals 34-1 and 34-2 are connected respectively to the opposite ends of the ground line 32, and each ground terminal 34-1 or 34-2, which are implemented by the COG method, is grounded through a respective one of the bump electrodes 36.
Under the COG implementation, not only is high contact resistance created, but also its deterioration is severe. With consideration of the deterioration, the value of the contact resistance varies greatly, such as from few Ω to few tens Ω. The resistance value of the ground line 32 in the scanning line drive circuits 30 is determined by the amount of an electromigration. Thus, in the case that an active matrix organic EL display panel as shown in FIGS. 4A and 4B is driven, it is assumed that the maximum allowable current is required to be 1.0 A of direct current. Here, the electromigration is the phenomenon that occurs when some of the momentum of moving electrons is transferred to nearby-activated ions. This causes the ions to move from their original position.
In the case of the assumption described above, since the maximum allowable current is generally set at 1 mA, the width W of the ground line 32 is required to be a 1000 m when the length L of the ground line 32 is set to be a 10,000 m. Since a sheet resistance is 0.05Ω/□, the resistance value of the ground line 32 having the length L is calculated to be 0.5Ω. As a result, as shown as an arrow in FIG. 4A, the output electric current, which is supplied from the left-end output terminal 33-1 to the ground line 32 through the switch 34-1, may flow to ground through the right-end ground terminal 35-2 in the case that the contact resistance at the ground terminal 35-1 is to be greater than the sum of the resistance value of the ground line 32 and the contact resistance at the ground terminal 35-2.
Further, the dispersion of the contact resistance can be reduced by the size of the ground terminal 35-1 or 35-2. However, if the sizes of the both ground terminals 35-1 and 35-2 are reduced, it would be required to form a hundred ground terminals near each ground terminal 35-1 and 35-2 to obtain the capacity to pass the electric current to ground. As a result, the size of the substrate 31 becomes larger because its longer side is further elongated. This is a distant idea in view of the difficulty of its implementation. Further, the width W of the ground line 32 may need to be a 1000 m so that the shorter side of the substrate 31 is also elongated. As a result, the total side of the scanning line drive circuit becomes larger. This is a specific problem with implementation using the COG method.
This problem cannot be solved by the technology disclosed in the above-described references. For example, according to FIG. 1 of the reference 1, the width of the wiring (107), which connects the two terminals (105) acting as the supply terminals to the current supply lines (104), is set to be a best value wherein two terminals (105) correspond to the ground terminals 35-1 and 35-2 and the current supply lines (104) correspond to the dale lines SEG and the wirings (107) correspond to the ground line 32. If the width W of the ground line 32 is set at the best value by using the technology disclosed in the reference 1, the ground line 32 having a 1000 m width is required as described above. Thus, the shorter side of the substrate 31 is elongated so that the problem is not solved.
According to the reference 3, the width of the power supply line supplying the power supply to the EL display panel is reduced to half so that the area of the flame part can be reduced wherein the area of the flame part corresponds to the area around the display area 10 shown in FIG. 2A. The reason that the width of the power supply line is reduced to half be that the power supply line extends from the mounting terminal area (102) at both of its sides, as shown in FIG. 4 of the reference 3. The technology disclosed in the reference 3 corresponds to the description above in that the width W of the ground line 32 is reduced to half by using two ground terminals 35-1 and 35-2 connected at the both ends of the ground line 32, as shown in FIG. 4A. Thus, the problem is not solved.
The reference 4 does not consider any width of the wiring, and the COG technology is not used. Thus, the teachings of reference 4 cannot be combined with the related arts to solve the problem.
As described above, the problem particularly occurring in the COG technology cannot be solved by the technology disclosed in the above-described references.